Production of high-cetane diesel fuel from low-quality biomass-derived feedstocks

ABSTRACT

A method is taught for producing diesel fuels of high cetane value from a triglyceride feedstock, comprising pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to partially convert the triglycerides and produce a middle distillates stream, and catalytically hydrotreating the middle distillate fraction to produce high cetane value diesel fuels. A biomass-derived diesel fuel is also taught having sulphur content below 10 ppm, a cetane-value of at least 70, a cloud point below 0° C. and a pour point of less than −4° C. A blended diesel fuel is also taught comprising 5 to 20 vol. % of the biomass-derived diesel fuel of the present invention and 80 to 95 vol. % of a petroleum diesel, based on total volume of the blended diesel fuel.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 11/234,175 filed Sep. 26, 2005.

FIELD OF THE INVENTION

The present invention relates to a two-step method for producing dieselfuel having a high cetane value from low quality biomass-derivedfeedstocks.

BACKGROUND OF THE INVENTION

In recent years, the area of biomass-derived diesel fuels has drawn agreat deal of attention. These fuels are plant and animal based and areproduced from such sources as canola, corn, soybean etc. Biomass-derivedfuels are generally environmentally less damaging to use thantraditional fossil fuels.

Another potential source for biomass-derived diesel fuels is from thewaste greases of animal rendering facilities and waste cooking oils,such as those found as restaurant trap greases. However these wastegreases and oils tend to contain contaminants that must effectively beremoved before processing.

In the past, catalytic hydrotreating has been performed on triglyceridefeedstocks in an attempt to produce high-cetane diesel fuels. Examplesof such processes can be seen in U.S. Pat. Nos. 5,705,722 and 4,992,605,herein incorporated by reference.

The cetane value of a diesel fuel is a measure of how easily the fuelwill auto-ignite at predetermined pressure and temperature and is oftenused to determine fuel quality. However, large quantities of hydrogenare required for this process, which is a major operating cost in theproduction of biomass-derived diesel fuel by catalytic hydrotreating.Reducing the volume of hydrogen consumed in the process would make theprocess economics more favourable. As well, hydrotreating was found towork best for very high quality feedstocks, such as tallow, vegetableoils (canola oil, soya oil, etc.) and yellow grease. Lower qualityfeedstocks, such as restaurant trap grease were found to be difficult toconvert by catalytic hydrotreating, due to their heterogeneous natureand the presence of contaminants. These contaminants were found torapidly deactivate the catalyst, thereby reducing hydrotreating reactortime on stream, requiring large quantities of catalyst to be used, andincreasing operating costs. There is therefore a great need to findefficient methods of producing a high cetane value product from lowquality waste triglyceride feedstocks, such as restaurant trap greasesand other waste greases, which can be used as a diesel fuel or as dieselfuel blending stock. There is also a need to find efficient methods toreduce hydrogen consumption in the catalytic hydrotreating stage.

SUMMARY OF THE INVENTION

The present invention thus provides a method of producing diesel fuelsof high cetane values from triglyceride feedstocks, comprisingpretreating the triglyceride feedstocks by thermal cracking or rapidpyrolysis to partially convert the triglycerides and produce a middledistillates stream, and catalytically hydrotreating the middledistillate fraction to produce high cetane value diesel fuels.

The present invention also provides a biomass-derived diesel fuel havingsulphur content below 10 ppm, a cetane-value of at least 70, a cloudpoint below 0° C. and a pour point below −4° C.

In yet another embodiment, the present invention provides a blendeddiesel fuel comprising 5 to 20 vol. % of the biomass-derived diesel fuelof the present invention and 80 to 95 vol. % of a petroleum diesel,based on total volume of the blended diesel fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail withreference to the following drawings, in which: FIG. 1 is a flow diagramof a first preferred process for carrying out the present invention; and

FIG. 2 is a flow diagram of a second preferred process for carrying outthe present invention.

DEFINITIONS

-   Biomass-derived diesel fuel—a diesel fuel produced by catalytic    hydrotreating of biomass feedstocks and containing practically no    oxygen.-   Biodiesel—a diesel fuel produced from the transesterification of    biomass-derived oils with alcohol and containing oxygen.-   Cetane number—measure of the ignition quality of diesel fuel    obtained by comparing it to reference fuels or blends of reference    fuels of known cetane number in a standardized engine test. The    reference fuels are n-cetane, having good ignition quality (CN=100),    and heptamethylnonane, having poor ignition quality (CN=15).-   High cetane value—for the purposes of the present invention a high    cetane value is defined as a value of at least 70.-   Waste triglyceride feedstock—a triglyceride from waste sources such    as restaurant trap grease, waste from animal rendering facilities    and other waste oil and grease sources, generally having at least    some contaminants.-   Catalytic hydrotreating—a refinery process for catalytically    converting and removing sulphur, nitrogen and oxygen from fuels and    fuel feedstocks at elevated hydrogen pressures and appropriate    temperatures.-   Middle distillates—encompass a range of petroleum fractions from    kerosene to lubricating oil and include light fuel oils and diesel    fuel, generally having a boiling point in the range of 150 to 345°    C.-   Thermal cracking—the process of breaking down large hydrocarbon    molecules into smaller molecules under high temperature and    pressure.-   Cloud point—a measure of the ability of a diesel fuel to operate    under cold weather conditions. Defined as the temperature at which    wax first becomes visible when diesel fuel is cooled under    standardized test conditions.-   Pour point—the lowest temperature at which a fuel flows, when cooled    under standardized test conditions. Generally taken to be 3° C.    (5.4° F.) or 1° C. (1.8° F.) (depending on selected temperature    interval) above the temperature of the no-flow point at which a test    vessel of fuel shows no movement when applying a controlled burst of    nitrogen gas onto the specimen surface (ASTM D 5949).-   Rapid pyrolysis—a process of decomposing chemicals at very high    temperatures and in the absence of an oxidizing agent. Rapid    pyrolysis has very short residence times compared to other thermal    decomposition methods.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present process employs a novel combination of thermal crackingfollowed by catalytic hydrotreating to convert low quality triglyceridesfeedstock into usable biomass-derived diesel fuel. In the presentprocess, thermal cracking is used as a pre-treatment step beforecatalytic hydrotreating, to partially break down the triglycerides intolower molecular weight components and fatty acids, which can then easilybe hydrotreated to produce a diesel fuel having a high cetane value andlow sulphur content. As an alternate to thermal cracking, rapidpyrolysis of waste triglycerides can also be used in the present processand details of rapid pyrolysis are given below.

A flow diagram of the process steps and streams of a one embodiment ofthe present invention is shown in FIG. 1. A feedstock of low qualitytriglycerides is fed to thermal cracking unit 10. The feedstock can beany variety of waste biomass, including restaurant trap greases, wastegreases from animal rendering facilities and other forms of waste oilsand greases and low-quality vegetable oils. Preferably, the feedstock 18is restaurant trap grease and other low-quality feedstocks. Thefeedstock stream 18 can be heterogeneous in nature and can containwater, carbon particles and have oxygen content as high as 14 wt. % ormore.

In the thermal cracking unit 10, the feedstock 18 is partially convertedinto a mixture of fatty acids and lower molecular weight hydrocarbons.Thermal cracking is preferably carried out under mild crackingconditions which are defined as preferably an operating temperature inthe range of from 390 to 460° C., more preferably from 410 to 430° C.,and preferably an operating pressure of from 0 to 415 kPa, morepreferably from 205 to 275 kPa. Thermal cracking produces variousfractions including gases 24, naphtha plus water 26, middle distillate22, and residue 20.

In an optional embodiment (not shown), the triglyceride feedstock may befiltered to remove any macroscopic contaminant particles.

The middle distillate stream 22 makes up more than half of the thermallycracked product and has been found to have suitable characteristics forfurther hydrotreating. Middle distillates typically encompass a range ofpetroleum equivalent fractions from kerosene to lubricating oil andinclude light fuel oils and diesel fuel. In one embodiment of thepresent invention the middle distillates were found to have a boilingpoint range of from 150 to 345° C., and more preferably from 165 to 345°C. The middle distillates fraction was found to contain as much as 40%less oxygen than the starting triglycerides feedstock 18, resulting inless hydrogen being required in the subsequent hydrotreating step.

The middles distillate stream 22 is fed to a catalytic hydrotreatingunit 12 containing a catalyst to facilitate and enhance thehydrotreating process. This catalyst is a commercial hydrotreatingcatalyst such as, for example, nickel-molybdenum, cobalt-molybdenum ornickel-tungsten on a catalyst support. It is preferably a supportednickel-molybdenum catalyst. Known methods in the art can be used tomaintain activation of the catalyst, thereby lengthening the useful lifeof the catalyst.

Hydrogen 28 is also fed to the hydrotreating unit 12. The presentinventors have found that, by partially removing oxygen from the feed inthe thermal cracking pre-treatment stage, hydrogen consumption in thehydrotreating step decreases significantly. Typical hydrogen consumptionfor hydrotreatment of clean, high quality biomass feedstock, withoutthermal cracking, is in the range of 2.3 to 3.0 kg H₂ per 100 kg offeedstock. By contrast, hydrogen consumption during hydrotreating of thethermally cracked middle distillates stream 22 is only between 1.5 to2.0 kg H₂ per 100 kg of middle distillate feed 22 to the hydrotreatingunit 12.

It has also been observed that, when processing thermally cracked wastetriglycerides, hydrotreating can be conducted at lower temperatures thanthose required for clean, high quality biomass feedstock. Hydrotreatingtemperatures in the range of 330 to 400° C., and more preferably 350 to390° C., are used in the present invention, compared to at least 375° C.typically required for hydrotreating uncracked, clean biomass-derivedfeedstocks.

Hydrotreated product 30 can optionally then be fed to a separator 14 inwhich the product 30 is separated into a gas stream 35, a water stream36 and a liquid organic product stream 38. The gas stream 35 can berecycled back to the hydrotreating unit 12 as a hydrogen recycle stream32, or it can form a fuel gas by-product stream 34.

In a preferred embodiment, the separated liquid organic product stream38 is fed to a distillation column 16 to further separate diesel fuel 40from any paraffinic residues 42.

Naphtha 26 and gases 24 from the thermal cracking unit 10 and fuel gas34 from the hydrotreating step can optionally be sold as valuableby-products. The residue streams 20 and 42 are small and can bediscarded by well known means in the art. Stream 42 is much cleaner thanstream 20 and can also possibly be used as feedstock for petrochemicalapplications.

Catalytic hydrotreatment of the middle distillate stream 22 produces abiomass-derived diesel fuel having a cetane value of from 75 to 80 andsulphur content below 10 ppm. Oxygen content of the resultant dieselfuel, an indication of the extent of conversion of the feedstock todiesel fuel, was found to be in the range of 0.09 wt % or less, on thebasis of the weight of product diesel.

The biomass-derived diesel fuel of the present invention also exhibitsexcellent cold-flow properties. The cloud point of the fuel is as low as−1.4 to −2.5 ° C. and the pour point is −4° C. or less.

In a further embodiment, the biomass-derived diesel fuel of the presentinvention can be used as diesel blending stock to produce a high cetanevalue blended diesel fuel. Preferably the blended diesel fuel comprises5 to 20 vol. % of the biomass-derived diesel fuel of the presentinvention and 80 to 95 vol. % petroleum diesel, based on a total volumeof the blended diesel fuel. More preferably, the blended diesel fuelcomprises 10 vol. % of the biomass-derived diesel fuel of the presentinvention and 90 vol. % petroleum diesel, based on a total volume of theblended diesel fuel. The cetane value of the blended diesel fuel wasfound to be proportional to the quantities of biomass-derived diesel andpetroleum diesel used in the blend and was generally higher than typicalvalues of 40 to 50 for standard petroleum diesel. Cold flow propertiesof such a blended diesel fuel are improved by the addition of petroleumdiesel and are superior to those of the biomass-derived diesel fuelalone.

As mentioned earlier, the step of thermal cracking can optionally bereplaced by a step of rapid pyrolysis. This process is shown in FIG. 2.Rapid pyrolysis is a process of decomposing a chemical at very hightemperatures and in the absence of an oxidizing agent. Rapid pyrolysishas very short residence times when compared to thermal cracking.

In the present invention, rapid pyrolysis of triglycerides, morespecifically trap grease, was conducted at temperatures ranging from480° C. to 600° C. for approximately 2 seconds. The triglycerides 18 arefed to a fluidized bed reactor 44 which is preferably fluidized withsteam 46, although other suitable fluidizing media known in the art canalso be used and are encompassed by the present invention. Steam 46 maybe fed to the reactor at a ratio ranging from 0.5 to 1.5, relative tothe triglyceride feed stream 18. The preferred steam to triglyceridefeed ratio is 0.9.

Any known inert gas 48 can optionally be added to the reactor to purgethe reactor of free oxygen during pyrolysis. The inert gas 48 ispreferably nitrogen. A catalyst may also be added, and suitablecatalysts include, but are not limited to acid washed activated carbon,calcined sewage sludge solids and silica sand, such as silica alumina.The catalyst acts to enhance the selective cracking of triglyceridemolecules to largely free fatty acid molecules.

Sample data from rapid pyrolysis trials on a trap grease feedstock islisted in Table 1 below. The resultant pyrolysis products are shown inTable 2. TABLE 1 Rapid pyrolysis conditions Run ID 261 265 253Temperature (° C.) 511 575 580 Fluidizing media Steam Steam SteamSteam/Feed ratio ˜0.9 ˜0.9 ˜0.9 by weight N₂ purge/Feed ˜7 ˜7 ˜7 ratioby weight Catalyst Acid washed Sewage sludge Silica sand activatedsolids, carbon, calcined at 35 mesh minus 750° C. Gas phase contact ˜2˜2 ˜2 time (s)

TABLE 2 Rapid Pyrolysis Products 261 265 253 Gas 28.2 11.3 7.6 Liquid50.3 89.4 90.7 Solids (coke) 9.0 Trace 1.4 Total above 87.5 100.7 99.7

The liquid fraction identified in Table 2 above contains middledistillates 22 as well as naphtha 26 and some residue 20. The boilingpoint distribution of the liquid fraction was determined bythermogravimetric analysis (TGA) and is given in Table 3 below. Themiddle distillates yield is given in Table 4. These tables indicate thatrapid pyrolysis of triglycerides produces an even larger proportion ofdesirable middle distillates than thermal cracking. TABLE 3 Boilingpoint distribution of the liquid fraction (from TGA) 261 265 253 Naphtha(IBP ˜165° C.) 86% 10%  8% Middle distillate 12% 75% 64% (165˜345° C.)Residue (345° C. plus)  2% 15% 28%

TABLE 4 Middle distillate yield with respect to feed 261 265 253 Middledistillate (wt % of feed) 6% 67% 58%

The middle distillate fraction 22 produced by rapid pyrolysis was foundto have varying free fatty acids (FFA) content, depending on thepyrolysis conditions. These details are shown in Table 5 below: TABLE 5Fatty acids in the middle distillate fraction Run ID 261 265A 265B 253Pyrolysis Temperature(°C.) 511 575 575 580 Total FFA wt % 0.63 45.7045.50 33.17

It was noted that the largest middle distillates fraction was producedby rapid pyrolysis at a temperature of 575° C. As well, FFA content washighest for this temperature range. A preferred temperature range forrapid pyrolysis of the present process is therefore from 550° C. to 600°C. and a most preferred range is from 565° C. to 585° C.

The difference in middle distillates yield between the run at 575° C.and the run at 580° C. is thought to be due to the difference incatalysts rather than the small difference in temperature. Catalystderived from sewage sludge is less acidic than silica sand. Thus,although the run with silica sand produced a slightly larger liquidsfraction by deoxygenation, this was accompanied by higher coke andresidue formation, resulting in an overall lower level of middledistillates. Thus the sewage sludge appears to provide a preferredbalance between higher middle distillate yield and lower coke formation.

It has also been noted that middle distillates produced by rapidpyrolysis comprise about 0.3 ppm nitrogen, compared with 5200 ppmnitrogen content in middle distillates obtained by mild thermalcracking.

As well, total sulphur in the middle distillate obtained by mild thermalcracking was in the order of 500 ppm whereas that in the middledistillate obtained by rapid pyrolysis was 150 ppm.

The following examples better illustrate the process of the presentinvention:

EXAMPLE 1

Conversion of Restaurant Trap Grease into Biomass-Derived Diesel

Restaurant trap grease having an average density of 0.925 g/mL, and anoxygen content of 13.72 wt % was fed to a thermal cracking unit where itwas cracked at a temperature of 418.5° C. and a pressure of 300 kPa for40 minutes. Thermal cracking produced a gas stream, a naphtha stream, amiddle distillate stream having a boiling point in the range of from 165to 345° C., water and residue. The middle distillates stream made up63.0 wt % of the total cracked product and its oxygen content was only7.99 wt %.

The middle distillate stream was then fed to a catalytic hydrotreatingunit. Hydrotreating produced a biomass-derived diesel fuel having acetane value of 75.4, a pour point of −6.0° C. and a cloud point of−2.5° C. The diesel was found to have less than 10 ppm sulphur content,which is well within tolerable commercial limits.

EXAMPLE 2

Conversion of Yellow Grease into Biomass-Derived Diesel

Yellow grease is waste grease resulting for rendering of animal fat. Inthis case, yellow grease, having a density of 0.918 g/mL and an oxygencontent of 11.56 wt. % was fed to a thermal cracking unit in which itwas cracked at 411° C. and 100 kPa for 40 minutes. Thermal crackingproduced a product containing 68.6 wt % middle distillates (165° C.-345°C.), 7.0 wt % naphtha and the remainder gas, water and residues.

The middle distillate stream, which was found to have 8.29 wt % oxygen,was then fed to a catalytic hydrotreating unit. The resultantbiomass-derived diesel stream had a cetane value of 79.2, a pour pointof −4.0° C. and a cloud point of −1.4° C. The sulphur content of thediesel was found to be less than 10 ppm.

This detailed description of the process and methods is used toillustrate one embodiment of the present invention. It will be apparentto those skilled in the art that various modifications can be made inthe present process and methods and that various alternative embodimentscan be utilized. Therefore, it will be recognized that variousmodifications can also be made to the applications to which the methodand processes are applied without departing from the scope of theinvention, which is limited only by the appended claims.

1. A method of producing diesel fuels of high cetane value from atriglyceride feedstock, comprising: a. pretreating the triglyceridefeedstock by thermal cracking to partially convert the triglycerides andproduce a middle distillates fraction; and b. catalyticallyhydrotreating the middle distillate fraction to produce high cetanevalue diesel fuels.
 2. The method of claim 1 wherein the triglyceridesfeedstock is selected from the group consisting of restaurant trapgrease, animal fats, waste greases, low-quality vegetable oils andcombinations thereof.
 3. The method of claim 1 wherein the middledistillates have a boiling point in the range of from 160° C. to 345° C.4. The method of claim 1 wherein thermal cracking is conducted at atemperature of from 390° C. to 460° C.
 5. The method of claim 1 whereinthermal cracking is conducted at a temperature of from 410° C. to 430°C.
 6. The method of claim 1 wherein catalytic hydrotreating consumesless than 2.0 kg of hydrogen) per 100 kg of middle distillate fed to thehydrotreating step.
 7. The method of claim 1 wherein catalytichydrotreating is conducted at a temperature of from 330° C. to 400° C.8. The method of claim 6 wherein catalytic hydrotreating is conducted ata temperature of from 350° C. to 390° C.
 9. The method of claim 1wherein catalytic hydrotreating is conducted using a commercialhydrotreating catalyst.
 10. The method of claim 9 wherein the commercialhydrotreating catalyst is nickel-molybdenum, cobalt-molybdenum ornickel-tungsten on a catalyst support.
 11. The method of claim 1,further comprising filtering the triglyceride feedstock to removemacroscopic contaminant particles before thermal cracking.
 12. Themethod of claim 1, further comprising conducting separation aftercatalytic hydrotreating to produce a gas stream, a water stream and aliquid organic product stream.
 13. The method of claim 12, furthercomprising distilling the liquid organic product stream to furtherseparate diesel fuels from paraffinic residues.
 14. The method of claim12, further comprising the step of recycling the gas stream as hydrogenrecycle during catalytic hydrotreating.
 15. A biomass-derived dieselfuel having a cetane-value of at least 70, a cloud point below 0° C. anda pour point of less than −4° C.
 16. The diesel fuel of claim 15, havinga sulphur content of below 10 ppm.
 17. The diesel fuel of claim 15,produced by the process of claim
 1. 18. A blended diesel fuel comprising5 to 20 vol. % biomass-derived diesel fuel as described in claim 15 and80 to 95 vol. % petroleum diesel, based on a total volume of the blendeddiesel fuel.
 19. The blended diesel fuel of claim 18 comprising 10 vol.% biomass-derived diesel fuel as described in claim 15 and 90 vol. %petroleum diesel, based on a total volume of the blended diesel fuel.20. A method of producing diesel fuels of high cetane value from atriglyceride feedstock, comprising: a. pretreating the triglyceridefeedstock by rapid pyrolysis to partially convert the triglycerides andproduce a middle distillates fraction; and b. catalyticallyhydrotreating the middle distillate fraction to produce high cetanevalue diesel fuels.
 21. The method of claim 20 wherein the triglyceridesfeedstock is selected from the group consisting of restaurant trapgrease, animal fats, waste greases, low-quality vegetable oils andcombinations thereof.
 22. The method of claim 20 wherein the middledistillates have a boiling point in the range of from 160° C. to 345° C.23. The method of claim 20 wherein rapid pyrolysis is conducted at atemperature of from 480° C. to 600° C.
 24. The method of claim 20wherein rapid pyrolysis is conducted at a temperature of from 550° C. to600° C.
 25. The method of claim 20 wherein rapid pyrolysis is conductedat a temperature of from 565° C. to 585° C.
 26. The method of claim 20wherein the triglyceride feedstock is fluidized with steam.
 27. Themethod of claim 26 wherein the steam to triglyceride feedstock ratioranges from 0.5 to 1.5.
 28. The method of claim 27 wherein the steam totriglyceride feedstock ratio is 0.9.
 29. The method of claim 20 whereinan inert gas is used to purge any oxidizing agents during rapidpyrolysis.
 30. The method of claim 29 wherein the inert gas is nitrogen.31. The method of claim 20 wherein a catalyst is used during rapidpyrolysis to enhance the cracking of triglycerides to largely free fattyacids.
 32. The method of claim 31 wherein the catalyst is selected fromthe group consisting of acid washed activated carbon, calcined sewagesludge solids and silica sand.
 33. The method of claim 20 whereincatalytic hydrotreating consumes less than 2.0 kg of hydrogen) per 100kg of middle distillate fed to the hydrotreating step.
 34. The method ofclaim 20 wherein catalytic hydrotreating is conducted at a temperatureof from 330° C. to 400° C.
 35. The method of claim 34 wherein catalytichydrotreating is conducted at a temperature of from 350° C. to 390° C.36. The method of claim 20 wherein catalytic hydrotreating is conductedusing a commercial hydrotreating catalyst.
 37. The method of claim 36wherein the commercial hydrotreating catalyst is nickel-molybdenum,cobalt-molybdenum or nickel-tungsten on a catalyst support.
 38. Themethod of claim 20, further comprising filtering the triglyceridefeedstock to remove macroscopic contaminant particles before rapidpyrolysis.
 39. The method of claim 20, further comprising conductingseparation after catalytic hydrotreating to produce a gas stream, awater stream and a liquid organic product stream.
 40. The method ofclaim 39, further comprising distilling the liquid organic productstream to further separate diesel fuels from paraffinic residues. 41.The method of claim 39, further comprising the step of recycling the gasstream as hydrogen recycle during catalytic hydrotreating.
 42. Thediesel fuel of claim 15, produced by the process of claim 20.